1. Field of the Invention
The invention relates to thermoplastic polyimide (PI), and in particular to a thermoplastic PI with superior heat resistance and dimensional stability, suitable for fabrication of adhesiveless doubled-sided flexible copper clad laminate (FCCL).
2. Description of the Related Art
With recent rapid progresses in miniaturization and high integration of electronic devices using flexible printed wiring boards, there is an increasing demand for double-sided laminates to cope with the trend toward lighter and higher-density circuits. Conventional double-sided clad laminates include PI base film coated with adhesives such as epoxy or urethane resin on both sides. The use of adhesives, however, increases the thickness of the final device, making it undesirable for use in fine pitch circuits. Moreover, adhesives can cause curling or result in poor dimensional stability and solder resistance. To overcome these problems, adhesiveless double-sided clad laminate has been proposed.
There are various methods for fabricating adhesiveless double-sided clad laminate, one of which is direct lamination of tri-layer polyimide with two copper foils. The tri-layer polyimide typically includes a polyimide base film between two thermoplastic polyimide (TPI) layers for adhesive. The tri-layer structure can be formed by coating the TPI layer, PI base film, and TPI layer sequentially, as disclosed in U.S. Pat. No. 6,379,784, Japanese Patent Publication Nos. 2004-042579, 2003-071982, and 2002-240195. The coating quality and appearance are affected by many factors, such as the design of coating heads, fluid and physical properties of the coating materials, and B-stage baking conditions. Alternatively, the tri-layer structure can be formed by coating TPI layers on both sides of a PI base film, such as kapton E, or Apical NPI.
The PI base film can be subjected to surface treatment to improve adhesion to TPI layers. Typical surface treatment includes plasma treatment and coating amino-silane coupling agent, such as disclosed in Japanese Patent Publication Nos. 11-277699, and 08-230103.
In addition, the TPI layer can be modified by silane or epoxy to increase adhesion. For example, Japanese Patent Publication No. 2003-200527 discloses a silane modified polyimide fabricated by reacting polyamic acid or polyimide with an epoxy group-containing alkoxysilane. Japanese Patent Publication No. 10-000733 discloses an epoxy modified polyimide. A mixture of polyimide siloxane containing OH or COOH groups and aromatic epoxy is applied on both sides of a PI base film and laminated with two copper foils. After lamination, thermal annealing is required, e.g., 160° C. for 1 hour, to allow the reaction of OH or COOH groups with epoxy to increase adhesion, which increases processing time and costs.